Summary The Bell Textron 212 helicopter (registration C-GNHX, serial number30983) was being ferried from Bolton, Ontario, to Richmond, British Columbia. The recently purchased helicopter was being flown by the company's chief pilot with two passengers on board. At 1220eastern daylight time, the helicopter was at an altitude of 1500feet above sea level with an airspeed of 100knots, when there was a series of loud bangs immediately followed by severe airframe vibrations. The pilot had difficulty controlling the helicopter for the next 10to 15seconds. The pilot immediately lowered the collective, pulled back on the cyclic control and brought the engine throttles to idle. He regained control of the helicopter, but the banging and vibrations continued. Every time one of the advancing main rotor blades came forward, it would climb off track abnormally. The vibrations and banging became more severe as the flight continued. The pilot proceeded toward a large ploughed field for an emergency landing. As the airspeed decreased, the helicopter became more controllable, and a successful landing was carried out. There were no injuries to the occupants. The helicopter was substantially damaged from the in-flight vibrations. Ce rapport est galement disponible en franais. Other Factual Information The weather for the flight was good visual meteorological conditions and was not considered to be a factor in this occurrence. Records indicate that the aircraft was maintained in accordance with existing regulations and approved procedures. The pilot was certified and qualified for the flight. After landing, a post-flight inspection revealed that one of the main rotor blades had sustained damage. A small section of skin near the blade tip, aft of the spar doubler, on the lower surface of the rotor blade had debonded. The skin was raised and curled, but had not separated from the blade (see Photo1). The debonded skin measured 25inches by 2inches between stations263 and288. It was later discovered that several of the main rotor head components and transmission Debonded lower surface of rotor blade and repair area (blade resting upside down) had been damaged by the severe vibrations encountered during the flight. The damaged main rotor blade (part number 212-015-501-115, serial number A-3257) had accumulated 3251hours of flight time since new. The total service life for the blade was 4000hours. A review of the blade's service records indicated that it was manufactured by Bell Helicopter and entered service in December1996. In early 2005, the same blade had been damaged while the helicopter was parked in a hangar. The blade was then shipped to an authorized rotor blade repair shop. While paint was being stripped from the rotor blade in preparation for repair, deep corrosion pitting was discovered on the lower skin surface between stations 243and 262, just inboard of where the debonding later occurred on the 10June2005 flight (see Photo1). Because the pitting pattern exceeded the allowable limits, the repair shop proposed a repair procedure to Bell Helicopter and received approval. The repair procedure included removing the damaged skin and replacing it with a bonded external doubler. The trailing edge trim tab was also replaced. The skin-to-inner core bonding procedure required using a bladder and heater blanket tool. This tool ensures proper curing of the adhesive by applying heat and pressure to the area being repaired. This type of repair is performed regularly to repair damaged rotor blades. The bladder and heater blanket tool that was used covered the rotor blade from its tip to a point inboard of the repair area, which included the area where the debonding took place on the occurrence flight. The repair process called for the temperature to be controlled and monitored during the entire cure cycle. After the repair was completed, the blade was inspected by tap hammer in the repair area and all the way to the tip. The blade was then returned and installed on C-GNHX. As part of the investigation, records of the repair procedure were reviewed by the TSB Engineering Laboratory and Bell Helicopter, and it was verified that the procedure was performed in accordance with the standard recommended procedures. Following the repair using the bladder and heater blanket tool, the blade was in service for approximately four flight hours before the lower skin debonded on the occurrence flight at the spar doubler between stations263 and288. The debonded section between stations 263 and 288 was examined. There was a cohesive bond of the adhesive to the skin substrate. However, the original bonding adhesive used during the manufacturing process was not uniformly adhered to the spar doubler; only small remnants of the adhesive remained (see Photo2). The total amount of adhesion could not be accurately quantified because the adhesive on the skin and on the spar doubler had eroded. This erosion Close-up of blade tip with arrows showing adhesive residue on the spar doubler of the adhesive was caused by the airflow and environmental elements entering the debond area in the time frame between the skin debonding and the completion of the emergency landing. A scanning electron microscope examination of the debonded skin sample indicated that the adhesive, while remaining firmly attached to the skin, had replicated sanding marks from the spar along the adhesive to spar interface, clearly showing that most of the adhesive had bonded well to the skin, but not to the spar. There was little indication of any adhesive remaining attached to the spar surface. The sanding marks on the spar surface are indicative of the standard procedure used for surface preparation before applying the adhesive. Following this initial examination, preparation for repairing the blade was begun by cleaning the spar surface. However, during this process, the surface was altered, and bonding evidence was removed, preventing further analysis of the bond to the spar surface. Other sections of the blade were examined to determine the overall adhesive nature of the bond between the skin and spar surface. Further samples of skin were peeled from different sections of the blade inboard of the area damaged on the occurrence flight. The area where the previous repair had been completed showed sporadic cohesive separation on the skin side. All of the other samples removed indicated even adhesion between the skin and spar surfaces. During the examination of other sections of the blade along the spar doubler, two areas were discovered where the honeycomb inner core was crushed and had separated from the rear of the spar. The two areas were located between station211 and station224 and between station263 and station287. A layer of foam adhesive rests between the core and the spar (see Photo3). In the two damaged areas, the adhesive Honeycomb core crush and separation for spar foam was present at the spar, but there was no contact between the adhesive foam and the core. Bell Helicopter's Engineering Department determined that, even though there was a non-effective bond between sections of the honeycomb core and the spar, the skin-to-spar bondline would not have experienced static or fatigue failure during the service life of the rotor blade. The available records were examined to determine if this blade had incurred any additional damage during its service life. There was no indication of any additional damage.